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Liskov Substitution Principle

What is wanted here is something like the following substitution property: If for each object o1 of type S is an object o2 of type T such that for all programs P defined in terms of T, the behavior of P is unchanged when o1 is substituted for o2 then S is a subtype of T.1

Barbara Liskov

In layman's terms if you have a program that depends on a class A and without modification of that program you send a class B, by definition B is a subclass of A.

When developing application you need to make sure you understand what is the behavior that you rely upon and make sure that the subclasses that are inheriting from the superclass are keeping the same behavior so they comply with LSP.

One example of the violation of this principle is The Square-Rectangle Problem.

  1. Barbara Liskov. Data Abstraction and Hierarchy, SIGPLAN Notices 23, 1988.↩